1. Field of the Invention
The present invention relates to a drive system for changing the switching speed of power switching elements.
2. Description of the Related Art
There is a conventional technique to change the switching speed of the power switching elements which form an inverter electrically connected to an on-vehicle device such as a motor generator mounted to a vehicle. For example, Japanese patent publication No. 3052792 discloses such a conventional technique to change the switching speed of a switching element in an inverter based on an input voltage supplied to the inverter. Decreasing the switching speed of the switching element suppresses generation of surge noise, but increases electric power loss generated on changing the switching speed of the switching element. That is, in order to maintain the reliability in operation of the switching element, it is necessary to prevent the switching element from receiving an excess voltage of not less than a withstand voltage of the switching element. Therefore it is necessary to increase the switching speed, as high as possible, within a range where surge noise does not exceed its upper limit value in order to suppress the electric power loss generated when the switching speed is switched. Because of the upper limit value of such surge noise depends on an input voltage supplied to the inverter, it is possible to increase the switching speed as high as possible by optionally changing the switching speed according to the magnitude of the input voltage supplied to the inverter while preventing the input voltage supplied to the switching element from increasing too excessively.
The above inverter forms a high voltage system which is electrically insulated from a low voltage system in a drive system for changing the switching speed of the switching element, where the high voltage system and the low voltage system are mounted to a vehicle. In general, the low voltage system in the drive system manages various types of information such as an input voltage supplied to the inverter. Accordingly, it is necessary for the low voltage system to generate and supply the input voltage information and an instruction signal to change the switching speed of the switching element to the high voltage system. In order for the low voltage system to transfer such information and instruction signal to the high voltage system, the conventional derive system requires an additional insulation means such as a photocoupler.
In not only the above conventional technique, but also other systems capable of changing the switching speed of each of power switching elements, it is necessary to add an additional insulation means in order to transfer the operation signal (which serves as the instruction signal) to instruct the power switching element to change its switching speed from the low voltage system to the high voltage system.